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instr - Physics 54 Optical Instruments If automobiles had...

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Physics 54 Optical Instruments If automobiles had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. — Robert Cringley Cameras and projectors The camera is essentially a dark box ( camera oscuro = dark room) into which light is passed through a lens, forming a real image on the back wall where a light sensitive substance (film or photoelectric material) is placed. The prototype is shown. The camera is “focused” (so that the image forms in the right place) by manipulating the lens, either by moving it back and forth or by changing its focal length. Usually the object is beyond 2 f distance, and the image distance is between f and 2 f , so the image is reduced. Special close-up cameras can focus on objects at 2 f or closer. The projector is a camera in reverse. The object is an illuminated transparency, placed between f and 2 f distance. The image is formed on a screen at distance greater than 2 f , so it is enlarged (and inverted relative to the object). The eye The eye is like a small camera, with the cornea/lens system playing the role of the camera lens and the retina playing the role of the film. Shown below is a drawing of the eye. Object Image Lens PHY 54 1 Optical Instruments
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The cornea is a basically spherical transparent cap which provides most of the focusing power. Behind the cornea is a watery fluid called the aqueous humor, leading through the opening called the pupil (whose size is controlled by the iris) to the lens , a jelly-like object which serves as a fine-tuning device; its shape can be changed to increase or decrease the focusing power by means of muscles to which it is attached. After light passes through the lens, it moves through the vitreous humor, another watery fluid, and is focused on the retina . This is a membrane with nerves ending in light-sensitive rods and cones. The macula , and especially the tiny fovea at its center, is the most sensitive part of the retina. This is where the image is formed of the object of interest, and the eyeball is turned to make that object focus on the fovea. The rest of the retina provides surrounding background of less interest, not very well in focus.
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